Lamp 2

The second light is coming through the gate of the stairway. I used an Omni light here to get a soft illumination around this area. Like many other light sources in that scene, I defined a linear falloff to avoid a negative "accumulation" effect of light. This would increase the total exposure too much at the end. This kind of behaviour simulates reality much better.
Next: copy/paste and set the light type to "Square Spot". This gives us a better focus of the volumetric effect. We do not need a bulb simulation here, because it would not be visible from the cameras point of view (Fig.10 - 13).